Molecular Population Genetics of theSRKandSCRSelf-Incompatibility Genes in the Wild Plant SpeciesBrassica cretica(Brassicaceae)

Abstract

Self-incompatibility (SI) in plants is a classic example of a trait evolving under strong frequency-dependent selection. As a consequence, population genetic theory predicts that the S locus, which controls SI, should maintain numerous alleles, display a high level of nucleotide diversity, and, in structured populations, show a lower level of among-population differentiation compared to neutral loci. Population-level investigations of DNA sequence variation at the S locus have recently been carried out in the genus Arabidopsis, largely confirming results from theoretical models of S-locus evolutionary dynamics, but no comparable studies have been done in wild Brassica species. In this study, we sequenced parts of the S-locus genes SRK and SCR, two tightly linked genes that are directly involved in the determination of SI specificity in samples from four natural populations of the wild species Brassica cretica. The amount and distribution of nucleotide diversity, as well as the frequency spectrum of putative functional haplotypes, observed at the S locus in B. cretica fit very well with expectations from theoretical models, providing strong evidence for frequency-dependent selection acting on the S locus in a wild Brassica species.

The Evolution and Diversification ofS-Locus Haplotypes in the Brassicaceae Family

“…Indeed, in the class II SCR tree, the clustering of interspecific triplets consisting of one haplotype from each of B. oleracea and B. rapa, together with one putatively functional B. cretica haplotype, is particularly evident (Figure 3). In fact, the entire reservoir of class II haplotypes in cultivated B. oleracea can be found within a single B. cretica population (Edh et al 2009) and, thus, we apparently have not found any functional class II haplotypes not previously known from Brassica in the wild B. cretica. Several closely related pairs of B. cretica/B.…”

“…Moreover, SRK lineages from the cultivated B. oleracea and B. rapa are widely dispersed among lineages from the wild B. cretica. Together with the observation of almost equal levels of SRK sequence polymorphism in wild and cultivated Brassica species (Edh et al 2009), this suggests that domestication has not entailed any severe bottleneck, with subsequent reduction in the number and diversity of lineages, at the Brassica S locus (cf. Igic et al 2004).…”

“…Since the B. cretica haplotypes form the same dominance class-specific clusters as the other Brassica species, we suggest that the dominance pattern is the same in wild B. cretica as in cultivated Brassica, with dominant class I and recessive class II haplotypes. In the absence of crossing data, we previously assigned B. cretica SRK haplotypes into putative groups of haplotypes with identical SI specificity on the basis of DNA sequence similarity (Edh et al 2009). As revealed by the ML SRK kinase domain tree, clusters of similar haplotypes assigned to the same functional group are, in all cases except one, monophyletic (Figure 1).…”

“…Approximately 100 mg leaf tissue was used for whole-genome DNA extraction with a DNeasy plant mini kit (Qiagen) according to the manufacturer's instructions. Exons 4-7 of the kinase domain of SRK and parts of exons 1 and 2 of SCR were amplified and sequenced as described in Edh et al (2009). As S-domainrelated regions are also found in S-locus genes other than SRK (e.g., SLG; Stein et al 1991), we chose to work with the SRK kinase domain instead of the S domain to avoid inadvertent PCR amplification of nonhomologous regions.…”

“…Haplotypes were identified on the basis of total sequence identity, including both introns and exons, in BioEdit version 7.0.5.3 (Hall 1999) after alignment using the Clustal W algorithm (Thompson et al 1994). For SRK, the 46 haplotypes identified in Edh et al (2009) were used in the phylogenetic analysis. For SCR, only class II haplotypes could be amplified, and six class II haplotypes were used in this study.…”